如果資料表1的A欄位為NULL，就取資料表2的B欄位
先將兩個Table做left outer join成一個TBL_A，再做case when事務取F_2，如F_2為NULL則取F_3
select TBL_A.F_1, (case when TBL_A.F_2 Is Not NULL Then TBL_A.F_2 Else TBL_A.F_3 End) AS F_4
FROM
(select A.code AS F_1, A.country_cht AS F_2, B.country_chs AS F_3 from TEST1 A left outer join TEST2 B on B.code=A.code where A.code='my' or A.code='tw') TBL_A
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